The present invention relates to the field of printing, and more particularly to screen printing systems. Specifically, the present invention concerns an apparatus for curing or drying an ink layer applied to a workpiece by the screen printing system.
Printed patterns or indicia applied to articles of clothing, such as t-shirts, have become extremely popular. As the popularity of such clothing increases, the demand for high volume production also increases. In the past, a single printing press would be sufficient to meet the demand for certain articles of clothing. Now, technically advanced printing systems exist that require only minimal human intervention while maximizing the product output.
In addition, the degree of consumer sophistication has increased. Where once a single-color print on a single-color shirt was acceptable, now printed indicia on articles of clothing can consist of ten or more colors in complicated designs. Moreover, consumer demand for dark colored shirts with screen printed indicia has increased.
At one time, multiple color printing posed a significant impediment to the mass production of articles bearing printed indicia. However, the advent of the multi-station, turret-type printing press made high volume production of these articles of clothing highly feasible. A printing system of this type is depicted in FIG. 1. One such rotary printing apparatus 10 utilizes a screen printing process. The apparatus 10 includes a floor-mounted support base from which extends a shaft 12. A hub 13 is mounted on the shaft 12 for rotation in the direction of the arrow R and upward-downward movement in the direction of the arrow D. This rotary printing system 10 includes a plurality of stations 15.sub.a -15.sub.h. Each station includes a support arm 16 that is centrally mounted relative to the base 11 and shaft 12. The stations 15.sub.a -15.sub.h each include a print carriage 18 that is operable in a screen printing process. The print carriage is movably supported by a pair of guide head portions 19a and 19b relative to the corresponding support arm 16. Also mounted to the support arm 16 at each of the stations is a mechanism for clamping a silk screen frame, comprising an outer clamp 21 and an inner clamp 22.
Details of the silk screen printing portion of the rotary apparatus 10 are found in FIG. 2. As shown in this more detailed view, the print carriage 18 includes a flood bar 36 that applies ink to the screen and a squeegee 37 that draws the ink across and through the screen. These components are mounted to the guide head portions 19a and 19b. Movement of the guide head portions, and consequently the print carriage 18 is accomplished by way of a pair of guide shafts 32 that are supported at the free end of the station by a front support 33. The front support 33 is attached to the end of the support arm 16 at each station, such as station 15h. The opposite ends of the guide shafts 32 are supported at a rear support 31, which is also connected to the support arm. The guide head portions 19a and 19b are attached to a center support 20 that is configured for sliding along the length of each of the guide shafts 32, typically under pneumatic or electric power.
The rotary printing apparatus 10 also includes a plurality of screen frames 34 supporting a tensioned screen 35. The screen 35 carries the particular image to be printed on the article. The screen frame 34 is supported between the outer clamp 21 and the inner clamp 22. Each of the clamps carries a pair of clamping cylinders 23 that exert a downward force on the screen frame 34 to lock the frame in position. The clamps 21, 22 are attached to the support arm 16 through the front and rear supports 32, 31, respectively.
Referring again to FIG. 1, the rotary printing apparatus 10 includes a plurality of workpiece platens 25.sub.a -25.sub.j. Each of the platens is engaged to the hub 13 by a support arm 26. In operation, each workpiece platen 25 rotates and raises or lowers with the controlled movement of the hub 13. The apparatus 10 includes a controller 30 that governs the operation and movement of the hub, and therefore the workpiece platens 25. In addition, the controller 30 controls the linear movement of each print carriage 18 that cross the screen 35 at each of the printing stations.
In the typical rotary printing apparatus, such as the apparatus 10, the number of workpiece platens 25 exceeds the number of stations 15 by two. Thus, as shown in FIG. 1, a first workpiece platen 25.sub.a is situated at a loading station while the last platen 25.sub.j is at an unloading station. The remaining workpiece platens 25.sub.d -25.sub.i are in registry with corresponding ones of the stations 15.sub.a -15.sub.h. In operation of the rotary apparatus 10, a worker manually loads an item, such as an article of clothing, onto the workpiece platen 25.sub.a at the loading station 28. Generally contemporaneously, the worker removes the completed printed item from the platen 25.sub.j at the unloading station 29. The controller 30 then sequences the rotary assembly by rotating the hub 13 and therefore each of the workpiece platens 25 to the next subsequent station. The controller then directs the print carriage at each station 15.sub.a -15.sub.h to apply ink in a silk screen process to the workpiece. Typically, the ink at each station is a different color. In this configuration, a new workpiece is loaded and a finished product unloaded each time the controller 30 indexes the workpiece platens 25.sub.a -25.sub.j.
The controller 30 is electrically connected to actuators for each component at each station. The controller sends a predetermined, and often user-modifiable, sequence of activation and de-activation commands to these actuators to perform the various functions of the printing apparatus 10. For instance, when the platens are indexed to the next station, the controller provides a command to a motor or air cylinder driving the shaft 12 or hub 13. The motor is precisely controlled so that each workpiece platen 25 is precisely located beneath a printing screen 35 at each station. The controller next issues a signal to raise the hub 13 and consequently each platen 25.sub.a -25.sub.j so that the work in process carried by the platen is brought into direct pressure contact with the printing screen. Another signal activates each print carriage 18 to apply the ink layer to the workpiece substrate. The cycle time for each print carriage can vary depending upon the nature of the ink and the applied pattern. Once the last print carriage has returned to its home position, the controller sends a signal to retract the workpiece platens to prepare for indexing the platen to the next station.
In many types of silk screen process, such as printing on light colored substrates, only a thin layer of ink is applied to the workpiece substrate. In some cases, the absorbency of the substrate material eliminates the need for a separate drying apparatus. Moreover, exposure to ambient air is often sufficient to adequately cure or dry the applied ink. Each applied layer of ink has a drying time that is less than the cycle time for the screen printing apparatus 10. The amount of time that each ink application is exposed to ambient air depends upon how quickly the workpiece platens 25 are rotated from one station to the next and the print carriage 18 at the next station is activated.
Frequently, screen printing apparatuses augment the ambient air drying by interposing heated elements. In one installation, electrical resistance heating elements are situated in proximity with each station 15. The heating elements impart radiative and/or convective heat to the printed material between successive printing operations sufficient to cure the newly applied layer of ink. Overheating of the printed work may result in wrinkles, discoloration, shrinkage, and/or scorching of the applied ink and the underlying material. Consequently, it is important that the heat application be closely controlled when independent heating elements are being utilized.
In certain installations, the initial platen 25.sub.a at station 28 can be used as both a loading and an unloading station. With this approach, the worker simply removes a finished product from the platen 25.sub.a and installs a new item onto the platen. With this configuration, the station 29 can be a drying station. In certain screen printing operations, the ink is sufficiently cured during layer the drying time between successive stations to accept a subsequent application of ink. However, before the item is removed from the workpiece platen, a final cure or drying operation may be necessary at the station 29.
One such system is illustrated in U.S. Pat. No. 5,249,255. This system utilizes an infrared lamp and a blower to cure the ink on a printed item traveling beneath the portable curing apparatus. The amount of time that the substrate and ink layer is exposed to the curing apparatus again depends upon the cycle time for the overall rotary printing apparatus. This cycle time can be as short at 4-6 seconds or as long as 20-30 seconds in various commercial environments. Once the item is fully dried or cured, the platen 25.sub.j at the drying station 29 is indexed to the unloading/loading station 28, whereupon the finished product is removed by the worker.
As indicated above, many screen printing operations rely upon very thin layers of ink so that exposure to ambient air is sufficient for adequate curing. However, other applications require a heavier layer of applied ink. For instance, before printing on a black material, such as a black T-shirt, it is typically necessary to apply a heavy layer of white ink to completely cover up the black substrate. In this instance, simply exposing the thick applied layer to ambient air for the typical machine cycle time is not sufficient to cure the white ink prior to application of the next color. In this instance, more rigorous curing is generally necessary immediately following application of such a white layer before any other color is applied.
One approach to solving this problem is presented in U.S. Pat. No. 5,595,113. As disclosed in this reference, an individual curing apparatus is mounted directly to the guide head portions 19.sub.a and 19.sub.b of a printing station subsequent to the heavy ink printing station. The curing device described in this reference supplants the normal print carriage 18 that would be conveyed by the guide head portions 19a and 19.sub.b. Instead, reciprocation of these portions according to the normal sequence of the rotary machine causes the curing apparatus to be reciprocated over the workpiece platen and the thick ink layer to be cured. With this approach, the drying time for the thick layer is increased by the duration of an inking cycle.
One obvious problem with this approach is that a printing station is taken out of commission. Thus, in the example device shown in FIG. 1, an eight-station machine includes seven stations 15.sub.a -15.sub.h provided for applying one of seven different color layers. However, if the print carriage 18 from one of the stations is removed, one less color layer can be applied. Of course, in the system disclosed in the '113Patent, for each additional thick layer of ink to be applied, the print carriage of the next station must be replaced by a drying mechanism. It is not difficult to envision that a rotary printing apparatus with seven initial printing stations can be quickly reduced to an apparatus capable of only applying four different color layers where those layers are relatively thick. Adding printing stations to accommodate additional colors or printing layers is expensive. Moreover, space limitations may limit the size of the rotary printing apparatus, which therefore limits the number of available printing stations.
There is therefore a significant need in the printing industry for a mechanism that quickly and efficiently cures or dries even thick layers of applied ink in a continuous production system. In addition, the need exists for such a curing system that does not consume a printing station of the printing apparatus, or otherwise requires significant modification to the existing apparatus. A further need exists for a drying or curing system that does not adversely affect either the work in process or the printing screen material.